1. Field of the Invention
The present invention relates to a finder optical system provided in a photographic apparatus, e.g., a single lens reflex camera.
2. Description of Related Art
A finder optical system of a single lens reflex camera is configured to form (at the eyepoint) an enlarged image of an object image that is formed through a photographing lens system, formed onto a focal plane plate and inverted to produce an erected image via an image inverter optical member, such as a pentaprism, etc., for a user to view via an eyepiece lens system.
An eyepiece lens system that is used in such a finder optical system is required to have a high observation magnification, is required to ensure a sufficient eye relief (the distance between the eyepoint and the apex of the outermost surface on the eyepoint side of the eyepiece optical system), and the diopter is required to be adjustable, etc.
Since the eye relief is restricted by the size of the exit surface of the image inverter optical member, such as a pentaprism, etc., the eye relief typically cannot be made very long if attempts are made to obtain a high observation magnification. Accordingly, such a restriction in eye relief is disadvantageous for users wearing glasses in particular, who require a long eye relief.
Hence, there have been various kinds of finder optical systems proposed which provide a diopter adjustment function so that a favorable (in-focus) finder image can be observed by a user (who normally wears glasses) with his/her glasses removed. For example, in Japanese Unexamined Patent Publication Nos. 2001-324684 and 2007-328160, finder optical systems are disclosed in which the eyepiece optical systems thereof are configured of three lens elements, i.e., either a negative lens element, a positive lens element and a negative lens element, in that order from the object side, or a negative lens element, a positive lens element and a positive lens element, in that order from the object side, in which a diopter adjustment is carried out by moving the middle positive lens elements, each of which serves as a diopter adjustment lens element, in a direction along the optical axis thereof.
However, in the finder optical systems disclosed in Japanese Unexamined Patent Publication Nos. 2001-324684 and 2007-328160, since the amount of change in diopter relative to the movement amount of the diopter adjustment lens element (middle positive lens element) is small, a sufficient diopter adjustment range cannot be obtained. Furthermore, if attempts are made to forcedly increase the diopter adjustment range by increasing the movement amount of the diopter adjustment lens element, aberration fluctuations increase, thereby deteriorating the optical quality.